A common problem in many different fields is needing to know the properties of a material within an enclosed vessel. Such properties may include, for example, the height of a liquid in a tank. For example, where an air space is formed above the surface of liquid fuel present in the fuel tank of an automobile or airplane, knowledge of the shape of the tank and the height of the air-liquid interface from the tank bottom will allow one to calculate the amount of remaining fuel.
Where a plurality of stratifying liquids are present within a tank, it may furthermore be desired to know the height of each stratified liquid layer. For example, where water is mixed with hydrocarbon fuel intentionally, such as when seawater is used as ballast in oil tankers; or unintentionally, such as when water is present in a vehicle fuel tank or such as when groundwater seeps into tanks for fuel pumps at filling stations, it may be desired to know the height of fuel layer(s) as distinct from nonfuel layer(s) for accurate determination of remaining fuel.
Time domain reflectometry (TDR) applies radar techniques to transmission line theory to detect the location of impedance transitions or discontinuities at interfaces between different layers of materials. In TDR, an interrogation pulse transmitted from a transmitter is reflected from such an impedance discontinuity, and the reflected pulse is received by a receiver. It is possible to calculate the distance (range) to the impedance discontinuity that caused the reflection from the observed round-trip propagation time of the interrogation pulse.
FIG. 1 is a schematic diagram illustrating a probe 100 as disclosed in copending U.S. nonprovisional patent application entitled “System and Method for Accurately Measuring Fluid Level in a Tank,” having Ser. No. 12/243,511, filed Oct. 1, 2008. As is shown by FIG. 1, the probe 100 contains an elongated portion 110, a shaped arm 120, and a sensor 130. The elongated portion 110 is a coaxial tube having a hollow center. The elongated portion 110 is shaped and lengthened to allow for positioning within a fuel tank, wherein a distal end 112 of the elongated portion 110 extends toward a bottom of the fuel tank, in which the elongated portion 110 may be positioned. Having a hollow elongated portion 110 allows fluid to enter the elongated portion 110, via the distal end 112, into the hollow portion to enable fluid level determination.
A proximate end 114 of the elongated portion 110 joins a distal end 122 of the shaped arm 120. The connection between the elongated portion 110 and the shaped arm 120 is provided in a manner so as to allow the combination of the shaped arm 120 and the elongated portion 110 to create a waveguide for an electromagnetic pulse provided by the sensor 130. In addition, the combination of the elongated portion 110 and the shaped arm 120 provide a coaxial waveguide.
The shaped arm 120 may be filled with a dielectric such as Teflon®. The Teflon® fill is a solid dielectric. Use of a Teflon® fill serves at least two purposes. First, the Teflon® fill provides impedance matching, second, the Teflon® provides a means to prevent fluid ingression to a non-gauging portion of the probe 100, thereby eliminating unwanted reflections due to multiple fluid levels inside of the probe 100.
In accordance with probe 100, an interrogation signal is sent by the sensor 130 into a transmission line, wherein the transmission line includes the combination of the shaped arm 120, the elongated portion 110, and beyond the distal end 112 of the elongated portion 110. The transmission line has three sections. A first section of the transmission line is from an excitation source, such as the sensor 130, to a top of the probe 100, also referred to as the distal end 122 of the shaped arm 120 (also referred to as the beginning of the gauge-able area). A second section of the transmission line is from the top of the probe 100 (the distal end 122 of the shaped arm 120) to a bottom of the probe 100, also referred to as the distal end 112 of the elongated portion 110. The second section of the transmission line is also referred to as the gauge-able area. A third section of the transmission line is from the bottom of the probe 100 to the end of a transmission line that runs past the end, or distal portion 112, of the gauge-able area.
As discussed above, a TDR system may have a sensor unit (transmitter/receiver), a probe, and one or more connecting elements, such as an arm. The connecting elements may be, for example, a coaxial cable, or a coaxial waveguide. Reflections may occur unless the impedance of the arm is matched with the sensor and the probe. For example, the sensor may have an impedance of 50 ohms, and the probe may have an impedance of 86 ohms. Such impedance matching has been accomplished by filling the waveguide with a dielectric material with a known dielectric constant, for example, Teflon®. However, a dielectric filled waveguide connector may be relatively expensive compared with, for example, a coaxial cable. Therefore, it is desirable to provide a modular TDR system with low cost interchangeable connecting components.
One technical challenge involved with a modular TDR system is connecting modular components in such a way as to minimize signal loss while transmitting the interrogation pulse between the sensor and the probe. For example, if there is a signal discontinuity or impedance mismatch at the connection between successive modular components, the connection may generate unwanted reflections, and similarly, absorb or divert energy from the interrogation pulse before it reaches the target medium. Such an energy loss may be problematic, both for decreasing the signal to noise ratio of the interrogation pulse as reflected off the target medium, and for obscuring reflections from the gauge-able area with reflections outside the gauge-able area.
Signal discontinuities or reflection points along a transmission line may also occur where the transmission line is crimped or bent at a sharp angle, for example, an angle greater than 45 degrees. Even so, there are applications where it is advantageous for the TDR signal to traverse a path that takes sharp turns. In aerospace applications of TDR, where components may by necessity be constrained to fit within the aerodynamic boundaries of the vehicle, it may be advantageous to have the probe section attach to a connector section at a relatively high angle of incidence, for example, in the range of 45 degrees to 135 degrees. Such an angled connector is referred to herein as an elbow.
In aerospace applications, it may be particularly important for a TDR fuel probe to span the full height of a fuel tank. Therefore, the gauge-able region of the probe should begin at the very top of the tank and extend nearly to the bottom. Since in some application, the fuel tank extends to the very edge of the vehicle, extending the probe to the maximum height requires a probe connection at the end of a gauge-able area to be at a high angle of incidence. Therefore, it would be advantageous to connect the probe to the pulse signal chain at the very top of the tank, while minimizing unwanted reflections usually associated with such a high angle of incidence.
TDR may be used in applications where the material being monitored is inflammable, for example in fuel tanks. Inflammable material may be ignited by electromagnetic energy arcing across the gap (“spark gap”) between two conductors, such as the center and outer conductor of a coaxial wave guide, creating a spark. Therefore, care must be taken to ensure that a high energy electromagnetic signal traversing the TDR interrogation pulse signal path does not ignite the inflammable material. For example, energy from lightning striking a vehicle may be conducted along the TDR signal path into the fuel tank and arc across the gap between the conductors, potentially igniting vapor in the fuel tank.
Non impedance matching elbow connectors or joints are known in coaxial cabling systems. However, while such prior art elbows allow the signal to be diverted at a high angle of incidence, they do not provide for impedance matching or adequate spark gaps for TDR purposes. In addition, positioning such a connector at the top of a probe would effectively shorten the measuring range of the probe, as the gauge-able area of the probe would have to end before the elbow connector. If an elbow connector is not used, the TDR signal path would have to be more gradually curved inside the vessel being monitored, decreasing the effective gauge-able area of the probe by using a portion of the span of the vessel for gradually bending the transmission line to prevent signal loss and minimize unwanted reflections. Therefore, there is a heretofore-unmet need for an impedance matching elbow connector that is intrinsically safe in a fuel environment.